The long-term objectives of this proposal are to understand the mechanisms that regulate cell polarity during epidermal growth and regeneration. Cell polarity refers to the unequal distribution of cell-cell junctions, cytoskeletal structures and organelles within the cell, and is the most fundamental architectural feature of epithelial tissues. When cell polarity is compromised cells become aberrantly shaped, misoriented, and improperly layered, leading to aberrant control of proliferation, differentiation and tumorigenesis. In highly regenerative adult epithelial tissues such as the skin, dysregulation of stem cells is thought to be an underlying cause of epithelial cancers, so it is critical that cell polarity and tissue architecture is maintained as skin cells continually divide, differentiate and die. While the establishment of cell polarity is fairly well understood, less is known about how polarity is regulated during tissue regeneration. Because the mammalian skin is one of the most strikingly polarized and regenerative tissues in nature, it is an ideal system to study the regulation of polarity during regeneration. Using this system I have identified a novel mechanism controlling the spatial distribution of polarity proteins in epidermal stem cells undergoing mitosis. During mitosis, polarity proteins are internalized from the plasma membrane into cytoplasmic membrane vesicles, transported back to the plasma membrane during cytokinesis and relocalized in polarized manner. This mechanism is essential for the proper maintenance of tissue-wide polarity across the epidermis. The central aim of the independent/ROO phase of this proposal is to decipher the mechanisms that regulate cell polarity as cells undergo mitosis and how dysregulation of these pathways affect tissue architecture, proliferation and differentiation. Specifically, I will 1) elucidate the mechanisms controlling.the internalization of polarity proteins during cell division; 2) identify novel genes essential for polarity maintenance during mitosis; and 3) define how polarity is re-established in post-mitotic daughter cells. Deciphering the mechanisms that regulate polarity in proliferative epidermal stem cells will be fundamentally important to the long-term objective of understanding how defects in polarity maintenance can lead to human disease.